1. Field of the Invention
The present invention relates to an image processing apparatus, and more particularly to an image processing apparatus that is capable of suitably reducing the noise in an image signal through the application of an averaging operation on the image signal based on the density and contrast data pertaining to the image signal, the image signal being obtained from a continuous tone image.
2. Description of the Related Art
When a continuous tone image is reproduced on a photographic film, or the like, after transforming the continuous tone image to an electrical signal by means of a solid-state image sensing device such as a CCD (Charge Coupled Device) image sensor, the reproduced image may become corrupted by noise owing to the operating properties of the solid-state image sensing device. When the image signal is corrupted by noise, the reproduced image will appear rough or uneven, making the reproduced image not fit for viewing. Particularly, since human vision exhibits an approximately logarithmic characteristic with respect to brightness, even when the shadow areas in the high density regions and the highlight areas in the low density regions in the continuous tone image are corrupted by the same amount of noise, a person will perceive a higher noise level in the shadow areas in the high density regions, and thus experience a substantial degradation of the quality of the reproduced image. A known method for reducing the visible noise level in the reproduced image is an image processing scheme in which the image signal, after having been transformed into an electrical signal, is subjected to an averaging operation.
When density levels of the whole continuous tone image are averaged and the result of the averaging process is used to generate an output, a reduction of the visible noise level is achieved. However, this also leads to a reduction of the resolution in the highlight areas of the reproduced image. While human vision exhibits a characteristic of low spatial resolution with respect to the shadow areas in the continuous tone image, a high spatial resolution characteristic is exhibited with respect to highlight areas. In effect, while the visible noise level in the shadow areas may be suitably reduced, the resolution in the highlight areas deteriorates.
Accordingly, in an apparatus disclosed in U.S. Pat. No. 4,868,671 the above-mentioned averaging operation on the image signal is performed with respect to the shadow areas only. In other words, in this prior art, each of the image signals derived from the continuous tone image is compared with a predetermined reference value, which is chosen to lie between values derived for the shadow and highlight areas, and the apparatus is designed to perform the averaging operation on the image signals based on the result of this comparison. In this case, an image having no appreciable noise in the shadow areas is obtained from the averaged image signals. At the same time, there is no reduction in the resolution in the highlight areas since the averaging operation was not applied in these areas.
However, in the image processing apparatus disclosed in U.S. Pat. No. 4,868,621 mentioned above, in the shadow areas with high density levels, the process of averaging the image signals and the surrounding pixels causes the edges in the high-density regions of the image to become smoothed, thus resulting in a deterioration of the sharpness of the image.
Also, in the above-mentioned apparatus, the decision as to whether or not the image signal should be processed for averaging is made based on the comparison of the image signal with one threshold value or several threshold values. As a consequence, the reproduced image will exhibit abrupt changes in the smoothness or sharpness of the image in regions where the image is supposed to change gradually from shadow to highlight. In cases where the image has gradual changes in density, a problem arises in that the reproduced image will have an unnatural quality in regions with density levels close to the aforementioned threshold value or values.